PDF correction

The probability density function (PDF) correction is done by mapping
CFS forecast PDF to observed PDF. The observed PDF is defined using OIv2 analysis
for 1981-2006. The CFS PDF is defined using individual members (15 for each initial month)
from the hindcast for 1981-2006. The PDF mapping is a function of initial time and
target time, and is done for monthly mean and for individual members.